Currently we do the following dance after sweeping a span:
1. lock mcentral
2. remove the span from a list
3. unlock mcentral
4. unmark span
5. lock mheap
6. insert the span into heap
7. unlock mheap
8. lock mcentral
9. observe empty list
10. unlock mcentral
11. lock mheap
12. grab the span
13. unlock mheap
14. mark span
15. lock mcentral
16. insert the span into empty list
17. unlock mcentral

This change short-circuits this sequence to nothing,
that is, we just cache and use the span after sweeping.

This gives us functionality similar (even better) to tcmalloc's transfer cache.

benchmark            old ns/op     new ns/op     delta
BenchmarkMalloc8     22.2          19.5          -12.16%
BenchmarkMalloc16    31.0          26.6          -14.19%

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/119550043

Fixes #8530.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rsc
https://golang.org/cl/124440043

Mallocgc must be atomic wrt GC, but for performance reasons
don't acquirem/releasem on fast path. The code does not have
split stack checks, so it can't be preempted by GC.
Functions like roundup/add are inlined. And onM/racemalloc are nosplit.
Also add debug code that checks these assumptions.

benchmark                     old ns/op     new ns/op     delta
BenchmarkMalloc8              20.5          17.2          -16.10%
BenchmarkMalloc16             29.5          27.0          -8.47%
BenchmarkMallocTypeInfo8      31.5          27.6          -12.38%
BenchmarkMallocTypeInfo16     34.7          30.9          -10.95%

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/123100043

Perf builders show 3-5% GC pause increase with GOMAXPROCS=1 when marking with atomic ops:
http://goperfd.appspot.com/perfdetail?commit=a8a6e765d6a87f7ccb71fd85a60eb5a821151f85&commit0=3b864e02b987171e05e2e9d0840b85b5b6476386&kind=builder&builder=linux-amd64&benchmark=http

LGTM=rlh
R=golang-codereviews, rlh
CC=dave, golang-codereviews, khr, rsc
https://golang.org/cl/128340043

Fixes #8480.

This CL reapplies CL 114420043. This attempt doesn't blow up when encountering hidden symbols.

LGTM=minux
R=minux
CC=golang-codereviews
https://golang.org/cl/128310043

LGTM=rsc
R=gobot, dave
CC=golang-codereviews, iant, rsc
https://golang.org/cl/114420043

In retrospect this should've been a variable instead of a type,
but oh well.

LGTM=agl
R=agl
CC=golang-codereviews
https://golang.org/cl/129250044

When building golang, the environment variable GOROOT_FINAL can be set
to indicate a different installation location from the build
location. This works fine, except that the goc2c build step embeds
line numbers in the resulting c source files that refer to the build
location, no the install location.

This would not be a big deal, except that in turn the linker uses the
location of runtime/string.goc to embed the gdb script in the
resulting binary and as a net result, the debugger now complains that
the script is outside its load path (it has the install location
configured).

See https://code.google.com/p/go/issues/detail?id=8524 for the full
description.

Fixes #8524.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/128230046

Generated by a+c.

R=gobot
CC=golang-codereviews
https://golang.org/cl/130140043

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/127410043

Fixes #8512.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/130090043

bv.data is an array of uint32s but the code was using
offsets computed for an array of bytes.
Add a test for stack GC info.
Fixes #8531.

LGTM=rsc
R=golang-codereviews
CC=golang-codereviews, khr, rsc
https://golang.org/cl/124450043

Fixes #7760.

LGTM=iant
R=iant, remyoudompheng
CC=golang-codereviews
https://golang.org/cl/130720043

LGTM=dave, minux
R=golang-codereviews, dave, minux
CC=golang-codereviews, rsc
https://golang.org/cl/122570043

Fixes #8492

LGTM=alex.brainman
R=golang-codereviews, alex.brainman
CC=golang-codereviews
https://golang.org/cl/122200043

LGTM=minux, adg
R=golang-codereviews, minux, adg
CC=golang-codereviews
https://golang.org/cl/129180043

On the go.benchmarks/garbage benchmark with GOMAXPROCS=16:
                   old ns/op     new ns/op     delta
time               1392254       1353170       -2.81%
cputime            21995751      21373999      -2.83%
gc-pause-one       15044812      13050524      -13.26%
gc-pause-total     213636        185317        -13.26%

LGTM=rlh
R=golang-codereviews, rlh
CC=golang-codereviews, khr, rsc
https://golang.org/cl/123380043

E.g., here's the new "go build" output:

$ go build misc/cgo/errors/issue8442.go
# command-line-arguments
could not determine kind of name for C.issue8442foo

gcc errors for preamble:
misc/cgo/errors/issue8442.go:11:19: error: unknown type name 'UNDEF'

Fixes #8442.

LGTM=iant
R=iant, alex.brainman
CC=golang-codereviews
https://golang.org/cl/129160043

CC=golang-codereviews
https://golang.org/cl/129130043

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/123390043

In cgo, now that recursive calls to typeConv.Type() always work,
we can more robustly calculate the array sizes based on the size
of our element type.

Also, in debug/dwarf, the decision to call zeroType is made
based on a type's usage	within a particular struct, but dwarf.Type
values are cached in typeCache, so the modification might affect
uses of the type in other structs.  Current compilers don't appear
to share DWARF type entries for "[]foo" and "[0]foo", but they also
don't consistently share type entries in other cases.  Arguably
modifying the types is an improvement in some cases, but varying
translated types according to compiler whims seems like a bad idea.

Lastly, also in debug/dwarf, zeroType only needs to rewrite the
top-level dimension, and only if the rest of the array size is
non-zero.

Fixes #8428.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/127980043

Restore https://golang.org/cl/41040043 after GC rewrite.
Original description:
On the plus side, we don't need to change the bits on malloc and free.
On the downside, we need to mark objects in the free lists during GC.
But the free lists are small at GC time, so it should be a net win.

benchmark             old ns/op     new ns/op     delta
BenchmarkMalloc8      21.9          20.4          -6.85%
BenchmarkMalloc16     31.1          29.6          -4.82%

LGTM=khr
R=khr
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/122280043

Basically this cleanup replaces all the usage usages of strcmp() == 0,
found by the following command line:

$ grep -R strcmp cmd/dist | grep "0"

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/123330043

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/128170043

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/125170044

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/129040043

The file is used by assembly code to define symbols like NOSPLIT.
Having it hidden inside the cmd directory makes it hard to access
outside the standard repository.
Solution: As with a couple of other files used by cgo, copy the
file into the pkg directory and add a -I argument to the assembler
to access it. Thus one can write just
        #include "textflag.h"
in .s files.

The names in runtime are not updated because in the boot sequence the
file has not been copied yet when runtime is built. All other .s files
in the repository are updated.

Changes to doc/asm.html, src/cmd/dist/build.c, and src/cmd/go/build.go
are hand-made. The rest are just the renaming done by a global
substitution. (Yay sam).

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/128050043

This allows implementing address-of-global
as a pc-relative address instead of as a
32-bit integer constant.

LGTM=rminnich, iant
R=golang-codereviews, rminnich, iant
CC=golang-codereviews
https://golang.org/cl/128070045

Fixes #7453.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/127210043

This allows changing the addressing mode for constant
global addresses to use pc-relative addressing.

LGTM=rminnich, iant
R=golang-codereviews, rminnich, iant
CC=golang-codereviews
https://golang.org/cl/129830043

You talked me into it. This and other links should be updated
once the new import paths for the subrepos are established.

LGTM=minux
R=golang-codereviews, minux
CC=golang-codereviews
https://golang.org/cl/124260043

Add a clause to the doc comment for the package and a
paragraph in the compatibility document explaining the
situation.

LGTM=bradfitz, adg, rsc
R=golang-codereviews, adg, bradfitz, minux, rsc
CC=golang-codereviews
https://golang.org/cl/129820043

codeblk and datblk were truncating their
arguments to int32. Don't do that.

LGTM=dvyukov, rminnich
R=iant, dvyukov, rminnich
CC=golang-codereviews
https://golang.org/cl/126050043

See golang.org/s/go14customimport for design.

Added case to deps_test to allow go/build to import regexp.
Not a new dependency, because go/build already imports go/doc
which imports regexp.

Fixes #7453.

LGTM=r
R=r, josharian
CC=golang-codereviews
https://golang.org/cl/124940043

LGTM=r
R=r, rsc
CC=golang-codereviews
https://golang.org/cl/126060043

Adds internal/syscall package.

Fixes #8520

LGTM=r, agl
R=agl, rsc, r
CC=golang-codereviews, iant
https://golang.org/cl/123260044

Turns out to be unused as well.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews, khr
https://golang.org/cl/127170044

It's unclear why we do this broken double-checked locking.
The mutex is not held for the whole duration of CPU profiling.
Fixes #8365.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/116290043

Fixes #7757.
Fixes #8488.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/118690044

This function does not have a declaration/prototype in a.h, and it is used only
in buf.c, so it is local to it and thus can be marked as private by adding
'static' to it.

LGTM=iant
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/122300043